The OEM manifold will sit the turbo too close to the engine. Before you make an adapter plate, bolt the manifold to the engine and place the turbo underneath it to see if everything clears. Leave at least a half inch between the turbo and anything it's near. Visualize how all the charge pipes and downpipe are going to run.
As for the adapter plate itself...
Use some 1/2" mild steel and have it machined so the faces are absolutly flat (do this with the exhaust manifold flange and the turbo flange too) so there's no leaks. Start by making templates of the turbo flange and the exhaust manifold flange and trace the exhaust manifold flange on the plate and overlay the turbo template over this. Make sure that none of the bolt holes line up.
There's a few different ways you can bolt everything up. You can drill and tap the plate and use studs, however, if you do this, you MUST make sure the stud extends through the other side of the plate, then grind off the excess, and stake it on both sides of the plate or it'll back off. The other way would be to drill holes and have recesses machined in the plate so the heads of the bolts are under the plate face. If you do the latter, you'd have to use roll pins to keep the bolt from turning when you tightened the nut.
I've made plates using the studs, and it works fine, as long as you use grade 10.5 studs and stake both ends well, and tighten everything VERY tight. Do not use spring lockwashers- their temper will be cooked right out of 'em during the first warm up/ cool down cycle. The only lock washer that'll survive on a turbo manifold is a star washer or some other interference type. Cost of the steel, studs, bolts, washers, and machine work should be under $50.